Susanna Clarke is a master of phrasing and evokes mood and emotion with the best writers I've ever read. Jonathan Strange & Mr. Norrell is a word-lovers paradise and would be perfect for Anglophiles looking for an alt-history period piece rich with description and fascinating personalities. Having said that, there are two things that may prevent some readers from enjoying this book as much as others.
The first is that it's a milieu story. Milieu stories focus on the world in which the story takes place in. They emphasize the history, language, literature, culture, and personalities that fill that world, and push story and individual character development into the background. Milieu stories are notoriously lacking in plot and this book is no exception. Don't get me wrong, things happen, but the overarching story is so subtle as to be nearly imperceptible. Answering the question, "what is the book about" is challenging. It's about watching people move through a fascinating alternate England with a rich and living history. There are also some bits about the Napoleonic War and a rivalry between the eponymous characters. You could also argue that it's about magic returning to England, though the war, rivalry and returning magic are simple excuses to give the characters something to do and aren't really the focus. If you're a fan of milieu stories, you will love this one. If you're looking for a strong plot with a clear understanding of where the story is going, and aren't a fan of detailed descriptions of locations, behavior and history, take a pass.
The second is a writer's pet peeve, so take it with a grain of salt. Jonathan Strange & Mr. Norrell is rife with passive voice that regularly pulled me away from the beautiful descriptions Susanna Clarke sculpts throughout the book. The phrases "seem to" and "appeared to" are also overused to the point that you aren't sure what's actually happening in many scenes. In her defense, the story involves the sometimes subtle magics of faeries and illusions, and for those scenes the phrases are used appropriately. Unfortunately they leak into scenes based in solid reality where they simply don't belong and, once again, pull me away from the stunning turns of phrase she creates.
Keeping those two things in mind, Jonathan Strange & Mr. Norrell is a fascinating trip through a beautifully realized 1800s, magic-filled England. If that's what you're looking for, this book should be at the top of your To Read list.